The function of the ribosome in protein synthesis is directly related to the specific interactions of groups of ribosomal proteins, so oriented in the native particle that they comprise active sites for binding ligands such as initiation factors, elongation factors, transfer and messenger RNA's. The purpose of this project is to map the topography or spatial arrangement of ribosomal proteins in the 30S and 50S subunits of E. coli. The technique being applied to this problem is protein-protein crosslinking using a new bifunctional reagent, methyl 4- mercaptobutyrimidate. The use of this new reagent, which forms interprotein crosslinks by disulfide bond formation and hence is reversible, coupled with a new diagonal (two-dimensional) polyacrylamide/dodecyl sulfate gel electrophoresis technique, permits the rapid identification of numbers of protein-protein pairs. The accumulation of a sufficient number of such dimer relationships will permit construction of maps which locate each ribosomal protein in specific positions relative to each other. This information, together with other evidence on ribosome function, will greatly augment knowledge of the fundamental mechanism of protein synthesis.